Add Cfn Policy Validater Action

https://github.com/awslabs/aws-cloudformation-iam-policy-validator

Could be an interesting option, but would require manual configuration by the user, since each invocation of this tool requires CFT-specific parameters to be passed to specific templates.

Might not be a great fit for pipelines of arbitrary code.

Start with the linked aws-cloudformation-iam-policy-validator project and inspect how its CFT-specific parameters must be supplied for each invocation. The issue names no files or tests; done would require a concrete decision on whether and how this validator can be integrated for user-configured templates and arbitrary-code pipelines.

Written by the indexing model from the issue text.